Ferrites are iron oxides that possess magnetic properties comparable in some respects to the magnetic properties of ferromagnetic metals such as iron, cobalt, and nickel. Although the magnetic strength of ferrites tends to be weaker than that of the ferromagnetic metals, an important and distinguishing feature of ferrites is that they exhibit a dielectric or electrical insulating property. For this reason, ferrites are particularly well suited for applications where electrical conduction is to be avoided, for example in microwave control devices for radar and communication systems.
A ferrite is also a gyrotropic medium that can influence the propagation of an electromagnetic wave or signal. At high frequencies, including the microwave and millimeter-wave bands, a gyromagnetic interaction occurs between the magnetization of the ferrite and the magnetic field component of the electromagnetic wave traversing the ferrite. At a specific frequency that is proportional to the strength of the internal magnetic field, the interaction becomes resonant and the electromagnetic wave is absorbed by the ferrite across a narrow band about the resonance frequency. For microwave frequencies, the applied magnetic field required for resonances is usually greater than 1000 Oe. At frequencies away from the gyromagnetic resonance condition, the absorption becomes negligible but a dispersion effect remains in the wave. This dispersion causes a change in the velocity of propagation that produces phase shift in phase shifters and switchable circulators.
The amount of gyromagnetic interaction is proportional to the magnetization in the ferrite whether at resonance or away from resonance. Magnetization in a conventional polycrystalline ferrite structure exhibits hysteresis. The term hysteresis means that the magnetic state of the ferrite structure is not directly reversible. For this reason, the shape and stability of the hysteresis loop are of critical importance to device performance that depends on a variable magnetization at low magnetic fields.
Polycrystalline materials are dense and comprise many individual crystals usually, but not necessarily, of random crystallographic orientation. Modern polycrystalline microwave magnetic devices are commonly operated in a remanent state and are designed to accommodate the hysteresis loop phenomenon. An initial negative magnetic field pulse drives the device into reverse magnetic saturation and a second positive magnetic field pulse selects an appropriate magnetization level of a minor hysteresis loop such that when the second pulse is removed, the device settles into a desired remanent magnetization.
This technique suffers from several limitations. First, it requires a look-up table to determine an appropriate magnetic field pulse strength to cause the device to settle into a particular magnetization. Because polycrystalline materials are used, these devices suffer from high coercivity and therefore, energy is wasted when switching between magnetization states. In addition, the hysteresis loop is rounded instead of square and therefore, excessive energy is required to reset the device into saturation. Furthermore, the switching time between pulses cannot be reduced below several microseconds without high current drive pulses.
One method for greatly reducing the inefficiencies and uncertainties introduced by the hysteresis loops exhibited by polycrystalline devices is the use of single crystal ferrite structures. A single crystal material has distinct preferred directions of magnetization uniformly throughout the material and exhibits virtually no hysteresis in its magnetization curve. In single crystal devices the magnetization can be crystallographically aligned with the preferred directions, in other words along the "easy" axes, in order to eliminate, or nearly eliminate, the hysteresis loop. This leads to a device which exhibits negligible coercivity and therefore has a magnetization which is nearly directly reversible. For single crystal devices, departure from alignment with the easy axis increases the energy required to magnetize the material. An example of such a configuration, magnetized along the "hard" axis, is given in U.S. Pat. No. 3,257,629, to Kornreich et al.
FIG. 1A illustrates a prior art closed loop magnetic structure 20. Current I flowing through a coil 23 generates a magnetic field which induces a magnetization M in the structure. FIG. 1B illustrates an alternative method for magnetizing an open-loop ferrite structure 25. An external magnet 29 having north N and south S poles, generates a magnetic field 27 which induces a magnetization Min the ferrite structure 25. In FIG. 1C, a coil 30 or solenoid is employed to generate the magnetic field 27. The external magnet techniques of FIGS. 1B and 1C generally require large magnetic fields for inducing or changing the magnetization M of the open loop structures 25 shown, as compared to small magnetic fields for the closed loop structure of FIG. 1A.
Assuming that the structure is formed of polycrystalline material, the structure exhibits a magnetization hysteresis loop as shown in prior art FIG. 1D. The magnetization loop illustrated is magnetization .+-.M as a function of applied magnetic field .+-.H between positive and negative saturation levels .+-.M.sub.S. This hysteresis loop clearly exhibits coercivity which is characterized by the coercive field H.sub.c required for reversing the magnetization of the structure. For this reason, the magnetization of the structure is not directly reversible.
Assuming that the structure is formed of single crystal material cut along the easy axis {100}, the direction of magnetization (M) in the structure of FIG. 1A is uniform along lines 22. At each corner, the magnetization changes direction uniformly along a domain wall 21. Single crystal magnetic devices offer the advantage of negligible coercivity as shown in the magnetization (.+-.M) as a function of applied magnetic field (.+-.H) chart of FIG. 1E. Saturation is illustrated at regions 28B (positive magnetic saturation M.sub.A 51A) and region 28A (negative magnetic saturation M.sub.B 51B). Negligible coercivity is exhibited in region 26 between the two saturation points 51A, 51B.
Prior art FIGS. 2A, 2B, and 2C represent single crystal magnetic structures cut along the {100}, {111}, and {110} planes respectively. These devices would exhibit magnetiz curves similar to the curve of FIG. 1C. Note that by convention, when referring to a specific axis, square brackets [. . . ] are used, while a family of axes are referenced using angular brackets &lt;. . . &gt;. Similarly, when referring to a specific plane, parentheses are used (. . . ), while a family of planes are referenced using braces {. . . }. In FIG. 2A, FIG. 2B, and FIG. 2C, the &lt;100&gt;, &lt;110&gt;, &lt;111&gt;, and &lt;112&gt; designations are standard crystallographic designations for crystals of cubic symmetry, for describing the family of axes of single crystal orientations in space.
Several articles discuss the behavior of hysteresis loops of single crystal magnetic structures:
1) H. J. Williams, et al., "A Simple Domain Structure in an Iron Crystal Showing a Direct Correlation with the Magnetization," Physical Review, 75(1):178-183 (January, 1949). PA1 2) J. K. Galt, "Motion of a Ferromagnetic Domain Wall in Fe.sub.3 O.sub.4 ", Physical Review, 85(4):664-669 (February 1952). PA1 3) F. B. Hagedorn, et al., "Domain Wall Mobility in Single-Crystal Yttrium Iron Garnet", Journal of Applied Physics, Supplement to 32(3):282S-283S (March 1961). PA1 4) G. F. Dionne, et al., "A Ferrite Bonding Method with Magnetic Continuity", IEEE Transactions on Magnetics, MAG-22(5):620-622 (September 1986).
These studies are directed to the behavior and speed of regions of reverse magnetization, also referred to as domains, moving through a single crystal structure, and the resulting coercivities of the structure. Coercive fields H.sub.c as low as 0.02 Oe (oersted) with square and stable magnetization curves have been demonstrated in single crystal structures.
An article was published in 1986 related to the elimination of "shearing" effects caused by gaps present in a ferrite toroid:
The result is referred to as "hysteresis loop shearing" caused by gap demagnetization. The study investigated a method for reducing the adverse and harmful shearing effects of the air gap created when two separate sections of magnetic material are bonded together to form a magnetic toroid. High permeability iron metal powder is introduced into the gap as a bonding material to reduce the shearing caused by the demagnetizing effects of the gap, resulting in improvement in hysteresis loop squareness.